Why am I measuring smaller when a month ago I was measuring perfect for my pregnancy?

I had my last ultrasound on September 25th and everything was perfect. The fluid, the baby’s weight, size, and my date was confirmed as feb 12th. I just had a doctors apt last Friday and she said I am measuring no where near the 23 weeks I should be at. She scheduled me for another ultrasound for the 30th. Is this something I should be worried about? And what can cause this? And I even had a checkup the week before the last ultrasound and everything measured fine then too. They couldn’t tell last time what we were having because the cord was laying inbetween his/her legs so I am excited to be able to hopefully see this time but then also worried about what this could mean. My first daughter was only 3 days early and she weighed 5lbs 14.5oz. I know with her I had numerous ultrasounds due to a strange antibody I had and I can’t remember if any of them were due to measuring small so I am kind of worried. Please let me know your thoughts